T8034.hs:7:3: error: [GHC-83865]
    • Couldn't match type: F a0
                     with: F a
      Expected: F a -> F a
        Actual: F a0 -> F a0
        NB: ‘F’ is a non-injective type family
        The type variable ‘a0’ is ambiguous
    • In the ambiguity check for ‘foo’
      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
      When checking the class method: foo :: forall a. C a => F a -> F a
      In the class declaration for ‘C’